In general, the so-called shoe press is widely used in place of a roll press in dehydration pressing in the paper industry. Briefly stated, the shoe press is a device pressing a first surface of wet web to be pressurized with a press roll or the like while pressurizing a second surface with a pressure shoe having a prescribed width in the running direction through a press belt thereby dehydrating the wet web. While a roll press performing pressing with two rolls applies linear pressure to an object to be pressurized, the shoe press provided with the pressure shoe having the prescribed width in the running direction can apply area pressure to the object to be pressurized. When performing dehydration pressing with the shoe press, therefore, a nip width can be increased for advantageously improving dehydration efficiency. The press belt is endlessly formed by an elastic material such as thermosetting polyurethane.
FIG. 7 is a schematic sectional view of an exemplary conventional shoe press 70. Referring to FIG. 7, wet web 73 held between a top felt member 71 and a bottom felt member 72 is transported into the clearance between a press roll 74 and a shoe press belt 75, and dehydrated by pressure formed between the press roll 74 and the belt 75. Both ends of the belt 75 are fixed to discs 79 rotatably supported on both ends of an unrotating support 78 through bearings. The belt 75 rotates in a driven manner following rotation of the press roll 74 while sliding on a pressure shoe 76. The pressure shoe 76 set on the lower surface of the belt 75 applies pressure to a pressurizing region A-A′, and this pressure is regulated in response to the pressure of oil injected into hydraulic cylinders 77 set on the lower portion of the pressure shoe 76 through the support 78. A plurality of drains 80 are formed on the outer peripheral surface of the shoe press belt 75 along the peripheral direction of the belt 75 with a uniform depth, so that drained water is discharged from the shoe press 70 through the drains 80.
The dehydration ability of the conventional shoe press 70 is remarkably influenced by the depth of the drains 80 formed on the outer peripheral surface of the shoe press belt 75. In other words, a large quantity of water can be drained from the wet web 73 if the pressure formed between the press roll 74 and the belt 75 is high, while the drained water cannot be sufficiently discharged from the shoe press 70 if the drains 80 are shallow.
The dehydration ability of the conventional shoe press 70 is disadvantageously readily reduced in the vicinity of end pressurizing portions A and A′. This is because the own weight of the large-sized support 78 of metal as well as depression of the press roll 74 deflect a portion close to a central pressurizing portion C as in a support 78a shown in FIG. 8, and the pressure on the end pressurizing portions A and A′ is increased as compared with that on the central pressurizing portion C. In other words, a shoe press belt 75a is remarkably worn in the vicinity of the end pressurizing portions A and A′ as compared with the central pressurizing portion C due to this deflection of the support 78a to reduce the depths of the drains 80 in the vicinity of the end pressurizing portions A and A′, leading to reduction of dehydration ability for the wet web 73 in the vicinity thereof Therefore, the conventional shoe press 70 cannot uniformly dehydrate the overall wet web 73 but causes a problem such as web break in a papermaking step and deterioration of the quality resulting from nonuniform paper strength.
In consideration of the aforementioned circumstances, an object of the present invention is to provide a shoe press belt capable of uniformly dehydrating the overall wet web and a shoe press employing the same.